During the manufacture of certain constructions of communications cable, a core comprising a plurality of electrically insulated conductors is enclosed within a metal sheath. The sheath may be made of copper, copper clad or stainless steel, steel, aluminum and low tensile copper. When constructed of high tensile copper, copper clad or stainless steel, steel, zinc-coated steel, terne plate or other clad metal, the sheath serves a dual function of shielding and armoring the cable core. Shielding protects the cable core from lightning damage and from electrical disturbances when installed in the field. By "armoring" is meant that the sheath is designed to provide strength when the cable is subjected to crushing loads, for instance when buried, or as protection against being cut in two by things such as rocks or rodents. When the sheath is constructed of aluminum or low tensile copper, it serves only to shield the cable core. The sheath may be constructed of dual sheathed systems of aluminum and steel, in which case the steel sheath particularly is the outer of the two.
Conventionally, the sheath is formed from a continuous tape, which is often corrugated and which is wrapped longitudinally about the cable core. The longitudinal edge portions of the corrugated sheath intermesh to form an overlap seam longitudinally along the core. It is sometimes required to join the overlapped edge portions together, for instance by soldering or bonding, so as to increase protection to the core.
The step of soldering, bonding or otherwise mechanically joining the steel sheath is usually one of the most expensive steps in the entire cable manufacturing operation. For example, when the seam of the steel sheath is to be soldered, it is normally necessary to provide one or more layers or coatings of conductive metal on its surface to permit reasonable soldering rates, thus adding considerably to the cost of manufacturing. Further, because of limitations on soldering rates, even when the conductive coatings are utilized, the cable manufacturing process normally cannot be carried out as a continuous in-line operation. In the forming of the cable core, the soldering of the steel sheath and the application of a final outer plastic jacket must be accomplished in separate steps. Additional expense is involved by the necessity of providing a heat barrier in the cable to protect the cable core during the soldering operation, and the cost of solder wire to form the soldered seam.
With the advent of a waterproofing system for communications cable of the type shown, for example in U.S. Pat. No. 3,607,487, issued to M. C. Biskeborn et al. on Sept. 21, 1971, it is no longer necessary for the sheath seam to be joined to form the hermetic seal as long as longitudinal edge portions of the metallic sheath are overlapped and a closed seam produced by suitable forming or working of the metal tape.
In manufacturing such a cable with an unjoined seam, it has been found that unless the overlapped seam is formed properly, the cable jacket will fail. Specifically, it was found that an outer overlapping edge portion of the metallic sheath tends to rebound subsequent to its forming and protrude into the plastic jacket that was extruded around it. Thus, the edge portions of the sheath tend to create a pinched cable which has a distorted periphery, or notch, which is not substantially circular in configuration. Under the sidewall pressures exerted on the sheath by the cable installation process, the edge of the overlap can cause the jacket to be cut and peel off the cable in a process known as "zippering".
It would thus be advantageous if there were an apparatus for forming the edges of the metal sheath such that if it were to rebound subsequent to forming, the integrity of the thermoplastic jacket would not be compromised.
The foregoing problems of the prior art which deal with the forming of communications cable are overcome by the method and apparatus of this invention, in which the edges of the tape are preformed to allow the overlap to be intimately formed when the sheath is made, thus improving sealing.